Thanks a lot Remi.
"Definitely"? Does that make the 380 a late planer?
Do you have an idea about the differences in numbers. Lets say I can pump my formula on a plane in 9-10 knots of wind. When will the 320 and 380 then start planing.

What about the cruising speed of the Phantom in less than 8 knots (where the formula is dead boring ;-).

Starting planing should not be THE criteria...Is it so satisfactory to plane on a APOLLO with 8 or 6 kn wind if you have to paddle to come back to your car?
And, what is the speed of an Apollo compared to the 380 in the same 6 to 8 knots.
And finally, what is the wind speed making the apollo faster than the 380 on any course (triangle or long distance round trip).

One thing is sure anyway, you don't buy a phantom 380 to just go reaching!

Racing longboards plane fairly early; just not as early as formula boards. In my experience, if I can get going in 10 knots with a certain sail on a formula board, then it will take about 12 knots to be fully planing with the same sail on a longboard. I wouldn't expect much difference in the planing threshold between the 380 and the 320 (both about 2 knots higher than formula), but I would expect a slightly more gradual transition from non-planing to planing on the 380.

Of course, the main advantage of a longboard is the fact that it performs well BELOW the planing threshold as well as above it. A longboard can acheive a fast glide and powerful upwind performance even in 5 knot winds when formula definitely shlogs. As some others have said, that comes in handy when you're trying to get back to the beach after the wind dies, or you're dealing with gusty or obstructed wind.

Also, an often-overlooked feature of longboard performance is the ability to slide the mast track forward, tilt the board to leeward, and really surge on the power of the rail and the daggerboard. You may not be technically planing, but you'll be getting great speed and an intense feeling, kinda like when you're flying one hull on a catamaran. And you can do it in significantly less wind than it takes to plane.

Thanks a lot.
But, what is the general speed of the longboards in around 5 knots? Did anybody measure with a GPS? I saw the video with Remi clocking +10 knots in almost no wind on a Serenity - amazing.
I live in an area with gusty winds. I do sail formula and get a real kick from it in +10 knots of wind, but below this it's basically boring to schlog such a thing.

Per
It's hard to put exact numbers on such things - longboard in displacement mode, quality of the wind, difference between gusts and lulls, etc.
If you sail inland with flukey/gusty winds and you're heavy, then there's a chance that a displacement hull will bring you much more TOW and enjoyment. Yes the Formula will plane earlier given enough wind, but if it doesn't plane than it's a royal pain as you already know. Try before you buy, find/borrow an old longboard, or even better, Div2 or a Serenity. As I've said, it's not the numbers, but the general feel that's so much different.

With a Serenity and a CodeRed 11.0m2 sail, I can go as fast as twice the wind speed, i.e., in 5 knots wind, I'm travelling at almost 10 knots board speed. As fast as a Hobbie Cat 18 footer. Faster than a Div 2 board, no question.
In 7 knots wind, formula guys are planing and are faster than me at twice the wind speed, mainly because my board speed is topping at 12 knots. Below 7 knots, they shlogg at 5 knots board speed while I'm twice faster. In a gusty 5-10 knots wind drag race, it's becoming pretty challenging to predict who will be winning between the Serenity and the Formula.